This invention relates to transfer molding techniques for the production of fiber reinforced resin composite structures. In particular, it relates to processes and apparatus for molded, vacuum bag construction of fiber reinforced resin composite structures of various shapes.
Fiber reinforced resin composite structures have taken many shapes and forms, including tubes or conduits as employed in the chemical processing industry, flat sheets and compound shapes as employed in boat building and extensive curved shapes as employed in aerospace. In these constructions a fiber reinforcement is laid up against a mold to provide the desired shape. This fiber reinforcement may be pre-wetted with resin, forming a "pre-preg" structure. More often a dry lay up of fiber is placed upon a mold and then wetted through by resin. The resin is allowed to set and cure upon the mold to form the fiber reinforced resin composite structure. Critical problems involved in this process include the uniform distribution of resin into the fiber, the elimination of air voids and bubbles, elimination of areas not wetted by resin within the fiber structure, and reduction of the use of excess resin or waste of resin in the process. The apparatus and processes, used to make these structures therefore vary considerably depending upon the specific shape and form of the structure to be produced.
A common technique is the so called vacuum bag technique which has been used to form fiber reinforced plastic structures. In vacuum bag techniques, flexible sheets, liners or bags are used to cover a mold which contains the dry or wet fiber lay up. The edges of the flexible sheet are clamped or sealed against the mold to form a sealed envelope surrounding the lay up. A catalyzed liquid plastic or resin is introduced into the envelope, into the interior of the bag, to wet the fiber, and a vacuum is applied to the bag interior via a vacuum line to collapse the flexible sheet against the fiber and surface of the mold. The vacuum serves to shape the article to the mold, to draw the resin through the fiber mat, completely wetting the fiber, and to remove any air which would form voids within the completed article.
The vacuum is continued while the plastic wetted fiber is pressed and cured against the mold to form the desired shaped fiber reinforced plastic structure. Since the entire resin curing process occurs within a sealed bag, and a continuous application of vacuum draws off all fumes through filters, resin fumes from the process are prevented from escaping into the ambient work space.
My prior U.S. Pat. No. 5,052,906 and U.S. Pat. No. 4,902,215 disclose vacuum bag structures in which distribution media are placed within the vacuum bag, surrounding the fiber lay up, in order to enhance the uniform distribution of resin through the composite upon the application of a vacuum. In order to enhance the separation of the completed molded fiber reinforced plastic structure from the mold and the bag, and to reduce the chances of contamination of the surface of the structure, peel plies, in the form of porous thin sheets which do not adhere to resin, are provided between the distribution media and the fiber lay up.
In the cited patents, fiberglass reinforced laminates are shown that have very desirable resin to fiber ratios and uniform mechanical properties. The vacuum bag process produces high strength structures in very complex shapes including boat hulls. However, each article created in this manner requires a separate set up of vacuum bag and distribution media; while this is desirable for one of a kind production, it is not the most efficient method of molding repeated identical structures.
In early vacuum bag structures, such as British Patent No. 944,955, the reinforced fiber structure is laid upon a single cavity mold, the mold covered by a flexible sheet or vacuum bag and the outer edges of the sheet sealed upon the mold to leave a space containing the dry fiber lay up. The liquid resin is introduced via supply line located at the top center of the bag and the vacuum is drawn on the bag by a vacuum line located at a peripheral edge of the mold. The collapse of the bag under the vacuum presses and forces the resin and fiber against the contour of the mold. A viscous resin require squeezing or rolling to distribute the resin through the fiber.
Other patents have suggested channels to distribute the resin through the fiber including U.S. Pat. No. 4,312,829 to Fourcher and U.S. Pat. No. 2,913,036 Smith.
In U.S. Pat. No. 4,942,013 to Palmer, et al. a structure is shown in which the resin is introduced through an opening at one end of the mold and a vacuum is drawn from the other end of the mold, thus drawing the resin laterally across and then through the fiber lay up. A second fiberglass layer is required to aid in the distribution of the resin.